Just be careful; sharp pointy ends on plastic plant can ruin a betta fish's tail. If I want to get plastic plants, I try and get the ones with big flat leaves. I tend to avoid any that has long and pointed leaves.
Before I stick them in the tank, I go over it and trim off any plastic bits that might be potentially dangerous.
:S Betta fish's fins are really fragile so it's best to just take that little extra precaution, especially since they like resting against plants a lot. If their fins tear, you can run the risk of them getting infected with bacteria and receive fin rot.